A large number of electrocatalytic reactions are considerably influenced by the structure and property of the catalyst. Efforts are therefore increasingly undertaken to examine the structural properties of fuel cells, and in particular of the catalytic layers of the particular electrodes.
While the identification and characterization of electrochemical reaction products take place exclusively during operation of the fuel cell, examinations regarding the characterization of the catalyst generally take place only prior to, and sometimes after, operation of the fuel cell.
Various examination methods are already known from the literature, which serve to arrive at a better understanding of the relationships between the catalyst structure and the resulting electrocatalytic activity of a membrane fuel cell (PEMFC). Methods used for structural characterization include microscopic, spectroscopic and diffraction methods, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS). Methods may be classified as methods for bulk and/or surface characterization. Cyclic voltametry (CV) or electrochemical impedance spectroscopy are generally used for fuel cells, in order to determine the electrocatalytic properties of the examined materials.
Of the above-mentioned methods, transmission X-ray absorption spectroscopy (XAS) is one of the standard methods for determining the short-range order in molecules and solid bodies. It is therefore particularly suited to tracking changes in the catalyst structure or the catalyst oxidation state during operation, since it supplies detailed structural information about the immediate surroundings of the atom examined.
In transmission XAS, the ratio of X-ray intensity before and after passage through the sample is measured, and absorption is thus determined. From a physical perspective, the irradiation of a sample with X-rays results in an interaction, which is to say ionization of atoms or molecules due to absorption of the radiation results as a function of the energy content of the radiation. The neutral particles absorb one or more photons from a radiation field, which itself experiences weakening in the process, which is then detected.
During these measurements, the source used is advantageously X-ray radiation having variable energy. This allows for selection from the continuous spectrum, so that monochrome radiation having a defined energy content may be used, which is suitable for selectively ionizing certain atoms or molecules.
The ionization threshold specifically depends on the orbital from which the electron (photoelectron) is removed by the ionization, in addition, the interaction of the photoelectron with neighboring atoms changes the attenuation coefficient of the X-ray radiation in the vicinity of the ionization threshold, this change being directly dependent on the energy of the original ionized electron (photon), and is thus atom-specific.
Transmission XAS is a very simple method, however it requires the sample to be thin enough to allow at least a portion of the X-ray radiation to pass through. At a low X-ray radiation intensity, the samples would have to be extremely thin, for example in the range of one micrometer.
In the case of transmission XAS, both the interior of the sample and the surface thereof contribute to absorption. However, the contribution of the surface is generally much lower than that of the volume and can therefore generally not be separately determined. Ionization counters are used to determine X-ray intensity.
In fuel cell electrodes, the catalyst particles are generally the atoms that are of interest, and in particular the platinum particles. A transmission XAS measurement on a fuel cell in which the intensity of the radiation is measured after passing through a platinum-supported catalyst layer, for example, the energy dependence of the attenuation coefficient in the vicinity of the ionization threshold allows conclusions to be drawn about the surface adsorption compounds, the coordination numbers, and the neighbors of an adsorbed platinum atom.
The big advantage of the transmission XAS method is that these measurements can take place in situ, which is to say during operation of a fuel cell, so that the state of the catalyst in an operating fuel cell can be examined. A limiting factor that must be noted is that the transmission XAS method supplies information regarding a bulk property, while the catalytic material used, in particular, in a low-temperature fuel cell is, by nature, generally irregular, and is defined by way of particle size distribution and corresponding morphology.
In general, both an anodic catalyst and a cathodic catalyst are used in fuel cells. Without further modification, a direct transmission XAS measurement perpendicular to the cell plane of a fuel cell would thus include the superimposed signals of both electrodes. Thus, if the same catalyst, such as platinum, is used in the two electrodes, the measurement results would hardly be meaningful.
One deliberation is therefore based on the idea that a catalyst-free region should be created in one of the two catalyst layers of the electrodes, in the region of the X-ray, so that the measurement signal contains only the information for the catalyst on the other electrode.
In fuel cells, the cathode in particular is the object of many examinations, most potential losses are assumed to originate in the cathode, it is suspected that a better understanding of the states of the platinum atoms of the catalyst layer on the cathode side of a fuel cell will allow potential losses there to be minimized in the future.
In practice, in in situ transmission XAS examinations of a fuel cell that is in operation, a small circular region is removed from an electrode comprising the catalyst so as to produce information about the catalyst of the other electrode alone. It is known from [1] and [2], for example, to remove a region measuring approximately 0.2 cm2 from the cathode in order to obtain better information on the anode catalyst. This catalyst is made of standard Pt—Ru (1:1) with 20 wt % on Vulcan XC-72, for example. At 1.2 mg/cm2, the loading was selected considerably higher than is customary so as to generate a good signal-to-noise ratio.
Since the absorption edge of Ru has a different energy, transmission XAS examinations and evaluations are not impaired by the presence of Ru.
However, based on simulations, it was possible to show that removing the catalyst from the anode window on the anode side results in a considerable reduction of the membrane potential φ and in a considerable decrease in the local current on the cathode side in the direction of the center of the measurement window. Moreover, it was established that the characteristics of the local current on the surface of the cathode also exhibit a quite significantly irregular radial distribution. This would indicate that the catalyst particles appear to be in different states within the irradiated cathode window (measurement window), depending on whether they are located on the surface.
It may furthermore be concluded from this that the existing information, based on standard transmission XAS measurements for the characterization of the state and the structure of a catalyst, may potentially have to be called into question, since they, for the reasons mentioned above, cannot be representatively applied to the entire catalyst layer outside the spot, even though they provide overall information about the catalyst within the cathode window.
It is the object of the invention to provide a method for characterizing a catalyst in a membrane fuel cell in situ, which is to say during operation of a fuel cell, which allows more realistic information to be provided about the state and the structure of the catalyst of the entire catalyst layer of the fuel cell than has previously been possible according to the prior art.
It is a further object of the invention to make a fuel cell design available which is adapted to this method and at least partially optimized.
Transmission X-ray absorption spectroscopy (transmission XAS) is a particularly suitable method for allowing examination of the catalyst on a fuel cell to be carried out during operation. To this end, the standard XAS examination method for a cathode of a fuel cell provides for a portion of the anodic catalyst, generally platinum or a platinum/ruthenium mixture, to be removed in a small region, so as to create a transparent window for the X-rays impinging perpendicularly on the fuel cell and allow unimpaired access to the catalyst of the cathode, which is to say without further absorption by a catalyst on the anode. Optionally this means completely removing a corresponding portion of the anode, together with the catalyst. The generally circular catalyst-free window, hereafter referred to as the “anode window,” has a radius, which is denoted by Ramax. The subscript a indicates the anode. In terms of diameter, this anode window generally corresponds substantially precisely to that of the X-ray used in the XAS measurement.
In the standard measurement system, a monochrome X-ray is conducted almost perpendicularly onto the cell planes of the fuel cell, so that a region (measurement window) having an identical radius Rkmax=Ramax is irradiated on the cathode side (subscript K). Depending on the energy content of the X-ray radiation, the catalyst particles located in this region on the cathode side bring about absorption, and thus weakening of the X-ray, which is then detected. The detector is usually likewise limited to the region of the measurement window to ensure that only the X-rays weakened by the absorption within the measurement window are captured.
Within the scope of the invention, it was found based on simulations that, due to the removal of the catalyst on the anode side, this standard examination method disadvantageously results in the current distribution within the fuel cell being changed in the irradiated region on the opposing cathode side, in fact, it was possible to show that this results in an uneven distribution of the potential as a function of the radius of the measurement window. The local current on the cathode surface in the region of the irradiation (measurement window) exhibits a corresponding inhomogeneity since the removal of the catalyst from a portion of the anode surface severely interferes with the electrochemical processes that take place at the electrodes.
While the previously obtained measurement results in sum are characteristic of the catalyst in the irradiated region on the cathode side, they generally do not represent the current state of the catalyst particles on the entire cathode side. This, however, is the very information that was hoped to be obtained from this type of examination (XAS).
Based on simulation examinations, according to the invention, a novel design for a fuel cell comprising a first electrode and a second electrode is proposed, which can be used particularly well for in-situ XAS measurement and which advantageously has an essentially uniform distribution of the local current across the second electrode surface within the measurement region. In this way, it is possible to ensure that, by way of such a measurement, information about the catalyst of the second electrode is obtained, which corresponds to a characterization of all the catalyst particles on or in the second electrode that is considerably better than in the past, and which does not apply only to the measured region of the measurement window.